These investigations represent the continuation of our studies on the Paneth cell in the small intestine and are designed to yield quantitative information on the secretory/endocytic function of these cells both in vivo and in vitro. The secretion of lysozyme and the lysozyme content of Paneth cells in vivo and in vitro will be quantitated by radioimmunoassay-immunocytochemical techniques and correlated with a) their cellular age (nonrenewing cell population) during differentiation, b) their secretory and endocytic compartments as determined by quantitative ultrastructural morphometry, c) the effect of various secretogogues including gastroenter-pancreatic hormones, d) diferent metabolic and surgical states known to influence the Paneth cells content of lysozyme, e) the effect of chemical agents known to influence the release of lysozyme by macrophages/leukocytes, f) the existence of cationic proteins or other enzymes in the Paneth cell and their relationship to antibacterial activity, and g) the in vitro endocytic activity of isolated Paneth cells and the immunologic factors influencing their phagocytosis of recognizable tracers. The results of these studies should provide important information on the physio-pathological significance of Paneth cells and their possible role in immunological defense mechanisms in the gastrointestinal tract. These studies may also provide insight into the mechanisms governing lysozyme secretion by other mucosal surfaces (mammary gland, respiratory tract, lacrimal gland, and female reproductive tract) as well as the endocytic activity of other epithelial cells (pigmented cell of the retina, epididymis, and kidney proximal tubule cell).